Oily fish may reduce risk of rheumatoid arthritis

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Behind the Headlines

Tuesday August 13 2013

Rheumatoid arthritis causes joint pain and swelling

"Eating fish could halve risk of arthritis" is the encouraging news in The Guardian, as a Swedish study found that women who regularly ate high levels of oily fish were less likely to develop rheumatoid arthritis.

Researchers asked women about their diet at two time points a decade apart to assess their intake of long-chain n-3 polyunsaturated fatty acids (omega-3 fatty acids).

The researchers then followed up the women six years after their diet was last assessed to see if they had developed rheumatoid arthritis.

They found that women whose dietary intake of omega-3 fatty acids consistently exceeded 0.21g per day at both time points had a 52% decreased risk of rheumatoid arthritis compared with women who consistently reported a dietary intake of 0.21g per day or less.

This corresponds to at least one serving of oily fish a week, or four servings a week of lean fish, such as cod.

However, the way this study was carried out means that it can't prove that eating fish directly prevented women developing rheumatoid arthritis. Despite this, there are many health benefits from regularly eating oily fish, including a reduced risk of cardiovascular disease.

Oily fish

A healthy diet should include at least two portions of fish a week, including one of oily fish such as salmon or mackerel.

However, babies, children and women who are pregnant, breastfeeding or planning to have children should have no more than two portions of oily fish a week.

The rest of us can eat up to four portions a week. This is the advised maximum level to avoid overexposure to marine pollutants.

Where did the story come from?

The study was carried out by researchers from the Karolinska Institutet and Karolinska University Hospital, Sweden. It was funded by the Swedish Research Council and Committee for Research Infrastructure and the Karolinska Institutet, a medical university.

The study was published in the peer-reviewed Annals of the Rheumatic Diseases.

This story was generally well reported by the media, but The Guardian and the Daily Express headline writers could have been a little more precise. They both talk about "arthritis", which is an umbrella term that covers a range of conditions that cause joint pain and swelling. The study in question looked at rheumatoid arthritis, which is one of the less common types of arthritis.

What kind of research was this?

Rheumatoid arthritis is an autoimmune condition where the body's own immune cells start "attacking" the joints of the body, causing pain and inflammation. The small joints of the hands and feet are most commonly first affected.

In this cohort study the researchers wanted to know if there is an association between dietary long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs) and the risk of developing rheumatoid arthritis. But cohort studies cannot show causation.

We can't conclude from the results of this study that n-3 PUFAs are directly responsible for the reduction in risk seen. This is because it is possible that there are other factors (confounders) responsible for the association seen.

For example, it is possible that people who eat a healthier diet that includes more fatty acids also have other healthier lifestyle behaviours that may also reduce their risk of developing certain conditions, such as a healthier diet overall and taking more regular exercise.

What did the research involve?

The researchers studied 32,232 women born between 1914 and 1948 who were living in a region of Sweden.

The women completed questionnaires on height, weight, the number of children they had, educational level, smoking history, physical activity and the use of dietary supplements.

Women who were diagnosed with non-rheumatoid arthritic conditions, had extreme energy intake, died before January 1 or took fish oil supplements were not eligible for the study.

The women completed a food frequency questionnaire at two time points: 1987 and 1997. The researchers calculated dietary intake of n-3 PUFAs by multiplying the frequency of food consumption (mainly fish and seafood) by the nutrient content of age-specific portion sizes.

New cases of rheumatoid arthritis were identified using two registers: the Swedish Rheumatology Register and the Outpatient Register of the Swedish National Board of Health and Welfare. The researchers were interested in cases that developed between January 1 2003 and December 31 2010. This was so women who had arthritis at the start of the study would not be wrongly identified as new cases.

The researchers looked at whether there was a link between the risk of developing rheumatoid arthritis and n-3 PUFAs and fish intake. They adjusted for the following confounders:

cigarette smoking

alcohol intake

use of aspirin

energy intake

What were the basic results?

Of the 32,232 women included in the study, 205 developed rheumatoid arthritis during the period January 1 2003 to December 31 2010, an average follow-up of seven-and-a-half years.

Dietary intake of n-3 PUFAs was divided into fifths (quintiles). Women in the bottom quintiles ate 0.21g per day or less of n-3 PUFAs, according to the food frequency questionnaire in 1997.

An intake of n-3 PUFAs of more than 0.21g per day (reported on the food frequency questionnaire in 1997) was associated with a 35% decreased risk of developing rheumatoid arthritis compared with a lower intake (adjusted relative risk [RR] 0.65; 95% confidence interval [CI] 0.48-0.90).

The researchers calculated that 28% of rheumatoid arthritis cases could be avoided if everyone had an intake of more than 0.21g n-3 PUFAs per day.

They also found that higher dietary intakes of n-3 PUFAs further reduced the risk of rheumatoid arthritis until an intake of 0.35g per day was reached. After this level, no additional benefit was seen with a higher intake.

When women consistently reported an intake exceeding 0.21g per day (both in 1987 and 1997), this was associated with a 52% (95% CI 29-67%) decreased risk of rheumatoid arthritis compared with women who consistently reported a dietary intake of 0.21g per day or less.

The researchers also found that women who reported eating at least one serving of fish (either oily or lean) per week in both 1987 and 1997 had a 29% decreased risk of rheumatoid arthritis compared with women who ate less than one serving per week (RR 0.71, 95% CI 0.48-1.04).

How did the researchers interpret the results?

The researchers conclude that in this study, they have observed a "statistically significant inverse association between intake of dietary long-chain n-3 polyunsaturated fatty acids and rheumatoid arthritis".

They go on to suggest that "moderate consumption of fish is sufficient to reduce risk of diseases".

Conclusion

This is a well-designed cohort study that found an association between an increased dietary intake of long-chain n-3 polyunsaturated fatty acids and a reduced risk of rheumatoid arthritis in a cohort of middle-aged and older women in Sweden.

This study has many strengths, including:

it was prospective, meaning that information was collected as the study was being performed

it used a large sample of women taken from the general population

diet was assessed at two time points, both long before rheumatoid arthritis was diagnosed

But because this is a cohort study, we cannot conclude from its results that dietary long-chain n-3 polyunsaturated fatty acids are directly responsible for the reduction in risk seen. This is because of the confounding factors that could also potentially be responsible for the association seen.

Although the researchers adjusted their analyses for the lifestyle factors of smoking and alcohol intake, which are associated with the risk of rheumatoid arthritis, it is possible that people who eat a healthier diet that includes more fatty acids could also have other healthy lifestyle behaviours. This could include having a healthier diet overall (such as a diet with plenty of fruit and vegetables and low in saturated fats) and taking more regular exercise.

In addition, this study provides no information about whether dietary intake of long-chain n-3 polyunsaturated fatty acids is associated with a reduced risk of rheumatoid arthritis in men or younger women. Further studies are required to confirm whether long-chain n-3 polyunsaturated fatty acids really do reduce your risk of developing rheumatoid arthritis.

However, it is currently recommended that people should aim to eat at least two portions of fish a week, including one portion of oily fish. Babies, children and women who are pregnant, breastfeeding or planning to have children should have no more than two portions of oily fish a week.

Eating this amount of fish would provide more than 0.21g of long-chain n-3 polyunsaturated fatty acids, which was the level associated with a reduction in the risk of developing rheumatoid arthritis.